The present invention relates to a vacuum interrupter applicable to use for switches employed in power plants, transformer substations and the like plants or stations.
Generally, vacuum interrupters are provided with an cylindrical insulation container made of alumina porcelain whose both end openings are sealed hermetically with sealing metals so as to allow the internal pressure to be reduced below 1.times.10.sup.-2 Pa. In the vacuum container a pair of contacts (electrode) are disposed so that these electrodes contact detachably to each other. The surface of both end openings of the alumina porcelain is provided with a metallized layer formed through coat-baking a powder of Mo-Mn or the like material thereon so as to make possible brazing between the surface and the sealing metals, respectively. Incidentally, the seal-brazing between the insulation container and the sealing metals is carried out at a temperature of 780.degree. to 1000.degree. C. In addition, an corrosion resistance agent is coated over the sealing-metal surface.
As is well known, the vacuum interrupter is required to be highly reliable. In particular, since the interior of the interrupter on operation must be kept at a highly vacuum state for a long time, it is necessary to take much-care over the sealing portion. Namely, at the conjunction between the insulation container and sealing metals, two substances different in the thermal expansion coefficient contact with each other. Therefore, the difference between their thermal expansion coefficient to be generated on brazing in a high temperature range, such as 780.degree. to 1000.degree. C, causes internal stress which can not be ignored. Accordingly, amelioration of the internal stress is now considered as one of countermeasures to enhance the reliability of the vacuum interrupter.
To solve the problem, the material for constituting the sealing metal has been selected so far from alloys, such as 42Ni-Fe and 17Co-29Ni-Fe alloys, having a thermal expansion coefficient on brazing close to that of alumina porcelain.
However, the conventional vacuum interrupter mentioned above still includes inconveniences as described below.
First, the countermeasure to corrosion over the interrupter body, especially the corrosion resistance treatment on the surface of the sealing metals should be further improved. Namely, the material having been used for such a treatment is an organic resin or the like coating film. The coating film, however, is likely to be deteriorated with time in quality and strength or in coating ability. Therefore, it is difficult for the vacuum interrupter employing such an instable corrosion resistance coating to guarantee a desired long-term operational reliability. Especially, in chemical works or in environments near the see, it is almost impossible for the instable coating film to prevent corrosion by chlorine gas or the same ion and to realize a long-term reliability of the vacuum interrupter.
Secondly, since the sealing metal is a ferromagnetic substance, temperature increase is caused by iron loss due to the operating current. Moreover, noise is generated by the magnetostrictive vibration.